FIG. 1 schematically shows a conventional mobile communication system. For example, the system may be a circuit-switched type mobile communication system employing the W-CDMA (Wideband-Code Division Multiple Access) method. As shown in FIG. 1, it is assumed that each of the user equipment (hereinafter may be referred to as a user equipment terminal) UE1, UE2, and UE3 is in communication with the base station apparatus (BS1) of the cell1 using the corresponding dedicated line assigned to the user equipment terminals. The base station apparatus may be referred to as base station (BS) or NodeB. In this case, a transmission signal of a user equipment terminal may become an interference signal for any other user equipment terminals and other base station (such as BS2 in the example of FIG. 1) as well. Therefore, it is necessary to adequately control the transmission power (more precisely, transmission power density),—more particularly, uplink transmission power density. As is generally known, by multiplying the transmission power (transmission power density) per unit bandwidth by the bandwidth, it is possible to calculate total transmission power assigned to the signal transmissions used in the bandwidth. It is power density rather than the total transmission power that directly interferes with the signals. Herein, basically, the term “power” refers to the term “power density” and unless any confusion arises, the term “power” may also be interpreted as “total power”.
In a conventional W-CDMA mobile communication system, the transmission power is controlled using a closed loop TPC (Transmitter Power Control) method (hereinafter may be simplified as “TPC”). In the TPC, a quality of a signal is measured at the receiver side, and the transmission power of the signal to be transmitted next time is adjusted so as to receive a predetermined quality of the signal by returning a transmission power control bit to the transmitter side. The transmission power control bit is transmitted via a return channel called DPCCH (Dedicated Physical Control CHannel).
In the system as shown in FIG. 1, the interference received by the base station (BS2) of a neighboring cell (Cell2) is estimated (determined) by summing the multiple signals output from the user equipment terminals UE1, UE2, and UE3. In the circuit-switched type communications, since a dedicated line is maintained for a relatively long period, the sum of the interference power from all the user equipment terminals is more likely to be rather equalized due to the statistical multiplexing effect. Therefore, it is expected that the transmission power can be stably controlled by using the closed loop TPC method.
Non Patent Document 1: 3GPP TS25.214